Greenish to reddish brown with thin dark longitudinal line and commonly iridescent blue rings and spot. Body size up to 5 cm. Produce purple ink when disturbed.

This is the largest species of sea hare, reaching up to 1.5 m in length. It has a sloping, disc-like shield edged with papillae, and a blunt, truncated-looking rear. A small groove in front of the shield contains the inhalant siphon that draws water into the mantle cavity.